This particular invention relates to dielectric compositions having very high dielectric constants (k), ranging between 8,000 and 19,000, and low dissipation factors (D.F.), e.g. below about 2.5%. Even more particularly this invention relates to multilayer ceramic capacitors (MLCC's) having high insulation resistance capacitance (RC), e.g., above about 10,000 ohm-farads at 25.degree. C. and above about 1,000 at 85.degree. C., and meeting Z5U/Y5V type temperature coefficient of capacitance (TCC) characteristics (Z5U=+22 to -56% over temperature range of +10.degree. to +85.degree. C., and Y5V=+22 to -82% over temperature range of -30.degree. to 85.degree. C.).
The novel dielectric formulations developed herein do not contain either Pb, Bi, Cd or Si (low melting point elements) and hence the MLCC's produced therefrom may be fired at high temperatures, e.g. 1360.degree.-1400.degree. C.
Because of their high volumetric efficiency and their small size, multilayer ceramic capacitors of the type described are the most widely used form of ceramic capacitors. Usually they are made by casting thin dielectric tapes, screen printing thereon the metal electrodes, and stacking the printed tapes to form the multilayer ceramic capacitors. There are other means of preparing MLCC's which are described in U.S. Pat. Nos. 3,697,950; 3,717,487, and 3,899,645.
Barium titanate powder is one of the major components most frequently used in the formation of dielectric insulating layers because of its very high dielectric constant (1,000). By selectively substituting the Ba and Ti sites with Ca, Sn, and Zr, one can shift the curie temperature pear to near room temperature, and hence powders having very high dielectric constants, greater than 7,000, may be obtained and also may meet Z5U/Y5V type temperature coefficient of capacitance. Higher dielectric constant, however, also leads to larger fired grain size; and larger grain size should be avoided when the dielectric powders are to be used in thin multilayer capacitor applications.
At present there are no satisfactory high fire (about 1400.degree. C.) Z5U type barium titanate based materials having a fine grain size as small as 2 um. Because the firing temperatures of typical low fire Z5U/Y5V MLCC's are in the range of 1080.degree.-1120.degree. C., several attempts have been made to reduce the sintering temperature of the MLCC's so that Pd/Ag alloys and/or mixtures thereof could be used as internal electrodes, and to enable the fired grain size to be controlled. (See for example U.S. Pat. Nos. 2,908,579, 2,626,220, 3,619,220, 3,638,084, 3,682,766, 3,811,937, and 4,640,905.) However, it is believed that high fire formulations have higher reliability over low fire formulations, possibly because high fire formulations do not have any low melting glass and network formers.
It is an object of this invention, therefore, to make high fire, barium titanate based Z5U/Y5V type dielectrics with uniform fired grain size in the range of 1-9 um, (uniform) [approximately 2 um for Z5U type dielectric and between 4 and 9 um for Y5V type dielectric formulations] while maintaining the dielectric constant as high as 8,000-19,000 and the dissipation factor as low as possible (less than 2.5%).
Another object of this invention is to produce a dielectric formulation containing no glass network formers such as Si, Na and K, and no low melting point elements such as Pb, Cd, and Bi.
A further object of this invention is to provide a dielectric material which may be used in thin dielectric layer MLCC aplications because of its fired grain size (2-5 um).
Yet another object of this invention is to produce a dielectric formulation with a very high dielectric constant, approximately 19,000, which can be used in normal dielectric layer thickness application meeting Y5V type dielectric spectrum, with fired grain size of approximately 9 um.
As noted hereinafter, the above stated objects are achieved by producing a product which may comprise a host material (Ba.sub.1-x Ca.sub.x) (Ti.sub.1-y-z Sn.sub.y Zr.sub.z) O.sub.3 as a major component, and several (one or more) minor components (dopants) of oxides of lanthanum, yttrium, zirconium, niobium and manganese, the total amount of dopants, not exceeding 2-3 wt. % and where the value of x is 0.10 to 0.14, y is 0.03 to 0.12, and z is 0.04 to 0.10.